Rotating machinery may be subject to unwanted vibrations due to the operation of the machine. For example, in four stroke engines, including diesel engines driving generators, each cylinder fires every other revolution. This causes the engine vibration to occur at ½ the engine RPM, called the ½ order vibration, and its higher order harmonics namely 1 order, 1½ order, 2 order. In the case of a diesel generator, in addition to the engine causing vibration the ½ order and its harmonics, the generator imbalance causes vibration at 1 order. Order in a machine is equivalent to the rpm of its rotating shaft. For example, 2 order means twice the rotating speed.
It is common practice to use a tuned vibration absorber to attenuate unwanted steady-state vibrations in such machinery. In this common method, a relatively small spring mass system is tuned to have its resonant frequency match the frequency of the unwanted machinery vibration. When attached to the machine, the added spring mass system will resonate 180° out of phase with the main body of the machinery and counteract the unwanted machinery vibration. This added spring mass system is sometimes referred to as an undamped vibration absorber since no damper is required in the system. Note that while the absorber will effectively reduce the unwanted vibrations at a target frequency, it does have the side effect of creating two new resonant frequencies in the combined dynamic system; one below the targeted frequency and one above the targeted frequency.
In a non-pneumatic tire and wheel assembly as described in U.S. Pat. No. 7,201,194, the assembly includes an outer annular band that contacts a road surface and supports the load on the tire. A plurality of web spokes function in tension to transmit the load forces between the annular band and a wheel or hub. Accordingly, a tire of the invention supports its load solely through the structural properties and, contrary to the mechanism in pneumatic tires, without support from internal air pressure. The structurally supported tire of the invention does not have a cavity for containing air under pressure, and accordingly, does not need to form a seal with the wheel rim to retain internal air pressure. The structurally supported tire does not require a wheel as understood in the pneumatic tire art. For the purposes of the following description, the terms “wheel” and “hub” refer to any device or structure for supporting the assembly and mounting to the vehicle axle or other device, and are considered interchangeable herein. Support forces are generated by tension in the web spokes not connected to the ground-contacting portion of the annular band. The wheel or hub can be said to hang from the upper portion of the tire. Preferably, the web spokes have a high effective radial stiffness in tension and a low effective radial stiffness in compression. The low stiffness in compression allows the web spokes attached to the ground-contacting portion of the annular band to bend for absorbing road shocks and to better conform the annular band to the irregularities in the road surface. To facilitate the bending of the web spokes of the ground-contacting portion of the tread, the spokes can be curved.
As an individual web spoke rolls through 360 degrees of rotation, it is subjected to tension forces, and then compression forces in the region of ground contact with the ground that cause a “buckling” of the spoke. Upon exiting the region of ground contact, the flexed spoke is suddenly extended or snapped back into a state of tension. This sudden extension is capable of exciting an unwanted vibration of the spoke. These unwanted vibrations may produce bothersome noise as the non-pneumatic tire and wheel assembly rolls, or may contribute to fatigue in the spokes themselves. The inventor has found that the steady state absorbers known in the art do not possess a damping mechanism that will give the desired result of reducing the unwanted spoke vibrations.
Therefore, it is desirable to have a non-pneumatic tire and wheel assembly that is capable of reducing these unwanted vibrations. The inventor has discovered a solution to this problem by a spoke configuration that includes an integral tuned and damped vibration absorber.